For measurements of modulation errors, measuring devices are already known, wherein an ideal reference signal is generated locally from a knowledge of the bits and the modulation parameters used for the digital modulation of the signal to be measured, such as type of modulation, filter function and symbol rate. The signal to be measured is then compared with the ideal reference signal (e.g. U.S. Pat. No. 5,001,724). In this context, the ideal modulated reference signal is generated in the measuring device either by synthesis or by calculation. With a modulation analyzer of this kind, the modulation error of a high frequency signal digitally modulated in any manner required can be measured and presented separately according to amplitude and phase as an I-error, a Q-error, vector-error amplitude or in a polar presentation as a vector diagram or configuration diagram.
One disadvantage of these known measuring devices is that when measuring signals, of which the amplitude or phase is distorted according to a known function on the digital side in the base band, for example, by being limited to a predetermined clip factor of 64%, a relatively high modulation error is already measured with reference to these signal distortions alone. If, for example, the modulation error of a transmitter power amplifier is to be measured with a known measuring device of this kind, and a distorted high frequency signal of this kind, for example, a clipped signal, is used for this purpose, the modulation error caused by the power amplifier, which is actually supposed to be measured, remains covered beneath the modulation error caused by the distorted measuring signal and can no longer be measured accurately. High frequency signals, which are distorted, for example, clipped, according to a known function, are increasingly used in modern radio systems for various reasons, and the modulation error can accordingly no longer be measured in the above manner. This applies in particular to clipped high frequency signals such as those used in the mobile telephone system IMT2000 of the next mobile telephone generation and/or in digital radio and television, where the modulation error of system components is supposed to be measured directly using the known measuring devices named above.